AP_ExternalAHRS: Remove GPS week comment

* It's a hold-over from MSP. None of AP handles 0xFFFF in a special way

Signed-off-by: Ryan Friedman <ryanfriedman5410+github@gmail.com>

libraries/AP_ExternalAHRS/AP_ExternalAHRS.h

@@ -137,7 +137,7 @@ public:
     } mag_data_message_t;
 
     typedef struct {
-        uint16_t gps_week;                   // GPS week, 0xFFFF if not available
+        uint16_t gps_week;
         uint32_t ms_tow;
         uint8_t  fix_type;
         uint8_t  satellites_in_view;